1. Field of the Invention
The present invention relates to a thermoplastic resin composition for molding a resin molding structure member included in electrical/electronic contact parts having a electrical contact such as a switch and a relay, and the electrical/electronic contact parts using the resin molding structure member using the thermoplastic resin composition.
2. Description of the Related Art
In general, an electrical/electronic contact parts such as switch and relay is required to exhibit a high fire retardance. At the same time, an electrical/electronic contact parts are required to comprise a minute and complicated resin-molded structure as one of constituent elements to attain miniaturization and weight reduction at which they are always aiming. Further, electrical/electronic contact parts are required to exhibit a prolonged electrical and mechanical life as well as prevent sticking (general term for fusion, locking and sticking between contacts).
As a resin molding material meeting these requirements, there has been normally used a thermoplastic resin composition obtained by blending a thermoplastic resin called so-called engineering plastics with various additives. For example, the engineering plastics is polyester resin, polyamide resin or polycarbonate resin having excellent mechanical and electrical characteristics. For example, various additives is organic halogen or phosphoric fire retardant (e.g., epoxy resin containing brominated bisphenol compound, pentabromobenzyl polyacrylate (PBBPA) and brominated polycarbonate oligomer).
However, if the conventional thermoplastic resin composition blended with various additives is used as a molding material for resin-molded structure constituting electrical/electronic contact parts such as relay and switch, an organic gas produced by the thermal decomposition of the resin or additives during and/or after the molding of the resin-molded structure undergoes mechanochemical reaction with the surface of a metal (e.g., Ag) at the electrical contact to produce a brown powder or reacts with the surface of the contact via arc to produce a black powder, thereby raising the contact resistance and hence causing contact failure. Thus, these problems are greatly disadvantageous to electrical/electronic contact parts essentially having requirements for high reliability. Further, the organic gas thus produced can undergo direct reaction with a mold heated to elevated temperatures to corrode the mold.
Among electrical/electronic contact parts, sealed electrical/electronic contact parts such as a relay and seal switch for a communication apparatus which have been sealed with a resin material for smaller size or enhanced reliability are remarkably disadvantageous in that an organic gas produced from the resin-molded structure cannot go out and stays in its inside to react with the electrical contact, junction or connection, thereby giving a rise in the contact resistance to cause contact failure.
In an attempt to solve problems such as contact failure caused by the generation of organic gas, it has been a common practice to use as a constituent element of electrical/electronic contact parts that a resin-molded structure which has been degassed by vacuum baking to eliminate the effect of the organic gas component produced from the resin-molded structure on the electrical contact.
However, the degassing by vacuum baking is disadvantageous in that it not only gives a reduced productivity that adds to the production cost but also impairs the gloss of the molded structure. Further, the resulting molded structure exhibits a reduced toughness and hence a high brittleness and thus can easily release a molding powder that stains the electrical contact to cause contact failure. Moreover, if the resin-molded structure is excessively degassed, sticking can occur. In an attempt to inhibit sticking, some approaches have been proposed, e.g., a method which comprises the application of a lubricant to the surface of electrical contact or sliding portion on electrical/electronic contact parts comprising a molded structure prepared by a molding process involving degassing by vacuum baking (as disclosed in Examined Japanese Patent Publication (kokoku) No. Sho. 63-5434, and a method which comprises the incorporation of a felt impregnated with a lubricant in electrical/electronic contact parts (as disclosed in Examined Japanese Patent Publication (kokoku) No. Sho. 62-195090). However, these approaches are disadvantageous in that they have to have additional steps and parts required for the preparation of electrical and electronic parts.
On the contrary, if the resin-molded structure is insufficiently degassed by vacuum baking, the remaining organic gas inevitably causes contact failure. Accordingly, in order to cause neither contact failure nor sticking when the conventional thermoplastic resin composition is used as a molding material, it is necessary that the baking degassing condition is optimized depending on the purpose of the molded structure. However, this countermeasure requires much time and labor and thus can be hardly conducted.
On the other hand, a resin composition which has a reduced volatile content to reduce the vacuum baking time is proposed in Unexamined Japanese Patent Publication (kokai) Nos. Hei. 6-9858 and Hei. 6-157881 taking into account a concept that the generation of the organic gas is mainly attributed to volatile content.
In other words, Unexamined Japanese Patent Publication No. Hei. 6-9858 proposes the use of a polybutylene terephthalate having a low terminal hydroxyl group concentration to minimize the generation of tetrahydrofuran (THF) on the basis of an assumption that the gas produced by the decomposition of the molded structure during molding or use comprises THF as a main component. Unexamined Japanese Patent Publication No. Hei. 6-167881 proposes the extension of the period of drying of the resin composition before molding to minimize the generation of volatile component. However, even these proposals can prevent neither the generation of contact failure nor the generation of sticking.
Further, the combined use of a halogenic fire retardant and an auxiliary fire retardant such as antimony oxide compound has been proposed as an approach for solving problems such as contact failure caused by the production of halogen gas instead of degassing treatment by vacuum baking. One example of the foregoing proposal is the use of antimony trioxide (Sb2O3) as an auxiliary fire retardant. Another example is the use of a double salt of antimony pentaoxide (Sb2O5) with an oxide of alkaline metal such as sodium oxide as proposed in Unexamined Japanese Patent Publication (kokai) No. 4-351657.
When such a double salt of antimony trioxide (Sb2O3) or antimony pentaoxide (Sb2O5) with an oxide of alkaline metal such as sodium oxide is used in combination with a halogenic fire retardant as an auxiliary fire retardant, the desired fire retardance can be given with a reduced total content of the fire retardant while reducing the produced amount of halogen gas as compared with the thermoplastic resin composition free of an auxiliary fire retardant even if no degassing by vacuum baking is effected.
However, the use of antimony trioxide (Sb2O3) as an auxiliary fire retardant is disadvantageous in that the absorption of halogen gas in the resulting thermoplastic resin composition is insufficient, thereby giving an insufficient effect of eliminating contact failure, and the electrical contact and the mold can be easily corroded as can be seen in the dry corrosion test described later. On the other hand, the combined use of a double salt of antimony pentaoxide (Sb2O5) with an oxide of alkaline metal, such as (Na2O)0.7.(Sb2O5)1.0 can provide an excellent absorption of halogen gas but is disadvantageous in that it gives a reduced adsorbed water elimination rate that reduces the melt stability and gives a great variation of melt viscosity with time, so that it is difficult to recycle products having stabilized properties. Further, this approach is disadvantageous in that γ-butyrolactone, which increases the contact resistance of the electrical contact, is produced. Thus, this approach leaves something to be desired in the elimination of contact failure.